[RFC PATCH v2 3/5] timers: Avoid blocking on hrtimer_cpu_base lock during Oops or panic

[Aaron Tomlin]

When sysrq_timer_list_show() dumps active high-resolution timers across
all online CPUs, it calls ktime_get() (which spins on tk_core.seq) and
print_active_timers() unconditionally acquires each CPU's
base->cpu_base->lock via raw_spin_lock_irqsave(). If invoked from an
Oops, NMI, or panic context while the crashing CPU or a wedged remote
CPU is inside a timekeeping write section or already holds
base->cpu_base->lock, sysrq_timer_list_show() will spin indefinitely
and prevent panic recovery or kexec from completing.

Use the NMI-safe ktime_get_mono_fast_ns() helper in
sysrq_timer_list_show() and raw_spin_trylock_irqsave() in
print_active_timers() when in_nmi(), oops_in_progress, or
panic_in_progress() is active, noting any contended lock and returning cleanly instead of
deadlocking.

Fixes: d999bd9392de ("panic: add options to print system info when panic happens")
Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
 kernel/time/timer_list.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 0406bf4488e9..4973650c4ae5 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -68,7 +68,15 @@ static void print_active_timers(struct seq_file *m, struct hrtimer_clock_base *b
 
 	touch_nmi_watchdog();
 
-	raw_spin_lock_irqsave(&base->cpu_base->lock, flags);
+	if (in_nmi() || oops_in_progress || panic_in_progress()) {
+		if ((!IS_ENABLED(CONFIG_SMP) && in_nmi()) ||
+		    !raw_spin_trylock_irqsave(&base->cpu_base->lock, flags)) {
+			SEQ_printf(m, "  [cpu_base lock contended]\n");
+			return;
+		}
+	} else {
+		raw_spin_lock_irqsave(&base->cpu_base->lock, flags);
+	}
 
 	curr = timerqueue_linked_first(&base->active);
 	/*
@@ -262,7 +270,8 @@ static inline void timer_list_header(struct seq_file *m, ktime_t now)
 
 void sysrq_timer_list_show(void)
 {
-	ktime_t now = ktime_get();
+	ktime_t now = (in_nmi() || oops_in_progress || panic_in_progress()) ?
+		      ns_to_ktime(ktime_get_mono_fast_ns()) : ktime_get();
 	int cpu;
 
 	timer_list_header(NULL, now);
-- 
2.55.0